In general, smelting refers to a process of reducing iron ores into pig iron in a blast furnace and steel making refers to a process of delivering molten pig iron, discharged from a tap of the furnace, to a converter to refine the molten pig iron (and remove impurities therefrom) thereby making molten steel.
Such a steel making process is divided into a first process of converting pig iron into molten steel and a secondary process of refining molten steel by controlling temperature and composition.
FIG. 1 illustrates the flow of general steel making and continuous casting processes. As shown in FIG. 1, pig iron produced by melting iron ores in a blast furnace is desulfurized and dephosphorized in an iron preparation process and delivered into a converter 10 for a converter process. In the converter process, pure oxygen is blown to the pig iron through an oxygen lance 12 to reduce carbon content to about 0.04% by weight, thereby producing molten steel removed of carbon.
Molten steel is tapped from the converter 10 into a ladle furnace 20 at a low temperature, by which its composition is controlled by reduction in P content, and then heated again.
Alternatively, by the use of molten steel heater 30 such as Chemical heating In Snorkel (CHIS) equipment and CAS-OP equipment, molten steel is heated with improved efficiency to delicately control the composition. This is novel secondary refiner that uses a snorkel 31 or an enclosed vessel sealed from the external air in order to cause chemical reaction between Al and oxygen, thereby raising the temperature of molten steel.
In addition, for the purpose of refining, molten steel may be delivered to Rurhstahl AG & Heraus Oxygen Blowing (RH-OB) equipment 40, one type of vacuum degassing refiners for the production of high purity steel. The RH-OB equipment 40 is another type of secondary refiner for extracting gases such as CO, nitrogen and hydrogen from molten steel through the backflow of molten steel in a vacuum vessel 41, controlling the temperature to enable continuous casting, and homogenizing the composition of molten steel.
Upon having been refined through the converter and secondary refining processes as described above, molten steel is supplied to continuous casting equipment 50, which makes slab from molten steel by continuous casting through a turn dish 51 and a mold 52.
FIG. 2 is a side elevation view illustrating a vacuum vessel 41 installed in a general vacuum degassing refiner. Referring to FIG. 2, the vacuum vessel 41 includes a top cover 41a, an upper vacuum vessel 41b, a lower vacuum vessel 41c and a snorkel 41d. 
FIGS. 3 (a) and (b) illustrate layouts of vacuum degassing refiners using two vacuum vessels 1 and 2 having such a structure, in which FIG. 3(a) is known as Japanese type and FIG. 3 (b) is known as European type.
It is referred to as a treatment position that the vacuum vessels 1 and 2 are arranged along the movement line of a ladle carriage 6 on which the ladle 5 filled with molten steel is loaded, and as a repair position that the vacuum vessels 1 and 2 are arranged along the movement line of repair carriages 7 and 8 for the purpose of the replacement or repair of the lower vacuum vessel and the snorkel.
That is, according to a conventional process performed in the vacuum degassing refiner as shown in FIGS. 3 (a) and (b), the ladle carriage 6 is driven to the treatment position to be located directly under the second vacuum vessel 2, the ladle 5 is raised to such a level that the bottom of the second vacuum vessel 2 is immersed with molten steel of the ladle 5, and then impurities are removed from molten steel.
Upon the completion of molten steel degassing, the ladle 5 is lowered from the raised position and seated on the ladle carriage 6, and then the ladle carriage 6 is driven to the tapping position for the tapping of refined molten steel.
During the vacuum degassing refining process as above, in case of Japanese type as shown in FIG. 3 (a), the lower vacuum vessels of the first vacuum vessels 1 and the snorkels are carried out to and in from a repair site 9a and the upper vacuum vessels fixed to vacuum vessel carriages 3 and 4 are repaired in the standby position.
In case of European type as shown in FIG. 3 (b), the snorkels are replaced with new ones by means of snorkel replacing carriages 3a and 4a in the standby position. The lower vacuum vessels are detached/attached by means of separate hydraulic equipment, and the repaired snorkels and lower and upper vacuum vessels are carried out to and in from a repair site by a repair crane 9.
The degassing refining has to be stopped in a process of replacing the snorkel or lower vacuum vessel in the treatment position or repairing the snorkel by the use of a spray gunning machine or hot frame gunning machine while the vacuum vessel is being repaired.
Thus, a long time is necessary to replace the snorkel or lower vacuum vessel with a new one or repair the snorkel. This, however, acts as major factors of shortening the work hours of the vacuum degassing refiner having limited average work times of 20 to 28, lowering productivity and raising manufacturing costs.